Servosystem with feedback discriminating means



United States Patent Ofifice 3,072,835 Patented Jan. 8, 1963 3,072,835 SERVOSYSTEM WITH FEEDBACK DISCRIMINATING MEANS Yutaka Maeda, Nagoya-shi, Japan, assignor to Kabushiki Kaisha Okuma Tekkosha, Nagoya-shi, Japan, a corporation of Japan Filed May 25, 1960, Ser. No. 31,694

Claims priority, application Japan Sept. 4, 1959 8 Claims. (Cl. 318-448) The present invention relates to a feedback circuit in an automatic control system and has for its object to provide such circuit having discriminating means for avoiding hunting to improve the stability of the automatic control system. It is known that, generally in a control system having a feedback, a dead zone, back lash or inertia effect acts to make the control system unstable. In the past, in order to overcome such difiiculties, various high-grade compensating means have been utilized or the gain of the control system has been properly adjusted to secure stability of the system. As is well known, increase of the gain impairs the stability of the system and renders its transient response oscillatory reducing the constant offset, while decrease of the gain improves the stability of the system increasing the constant offset thereof. Therefore, it is usual that the gain is decreased to secure a substantial stability at the sacrifice of sensitivity. The present invention has an advantageous feature that a discriminating means is provided in a control system for preventing the system from falling in the state of hunting while maintaining a high sensitivity.

Description will now be made on an embodiment of the present invention with reference to the accompanying drawings, in which:

FIGURE 1 is a block diagram showing the principle of a feedback circuit having discriminating means incorporated therein in accordance with the present inventien;

FIGURE 2 illustrates one embodiment of the present invention; and

FIGURE 3 is a graphic representation of the follow-up characteristic of the control system.

Referring to the drawings, particularly to FIGURE 1, there is diagrammatically shown a feedback circuit having discriminating means which operates when where designates an input or command voltage, 6,, a feedback voltage, and t time. I

Referring to FIGURE 2, a pair of variable resistors R and R having taps or sliding terminals a and b, respectively, are connected in parallel with each other and connected at opposite ends to terminals of a battery B, forming a bridge circuit having two Potentiometers. The sliding terminal a of the variable resistor R is, by way of example, arranged to move in association with the crossfeed rod C for driving the saddle of a lathe, while the sliding terminal b is arranged to move in association withthe movement of the shaft K of a speed change gear F for changing the rotational speed of the lathe spindle. An

amplifier means A is provided which receives as an input and amplifies the potential difference between said sliding terminals a and b. The output of the amplifier A is utilized to actuate a polarized relay CR or CR Either of the polarized relays is arranged to operate only when a current flows therethrough in the direction indicated by the arrow. A direction-discriminating switch means D is actuated in association with the movement in either direction of the lathe saddle to energize either the polarized relay CR or CR In more detail, it is arranged in the illustrated embodiment so that when the saddle is advanced one of the relays CR is energized while when the saddle is retreated the other relay CR is energized.

A servo motor SM is provided which is connected to an alternating-current source AC through a pair of contacting means CR or CR of the polarized relay CR or CR respectively, for rotation in one or the other direction. The servomotor SM also includes a shaft E connected with the shaft K of the speed change gear to supply a power to drive the shaft K thereby to move the sliding terminal b.

In FIGURE 3, there is graphically shown the followup characteristic of the feedback voltage relative to the input voltage.

In operation, as the feed rod C is turned to advance the saddle of the lathe for facing work, the variable resistor R is changed in its resistance value to destroy the voltage equilibrium between terminals or and b of the resistors R and R respectively. It is assumed in the embodiment that the input voltage 0 at the terminal a corresponds to the distance of a cutting blade from the spindle axis of the lathe, increasing with the advance of the saddle toward the spindle axis. On the other hand, the feedback voltage 0,, at the terminal b, corresponding to the rotational speed of the speed change gear, is assumed to increase as the rotational speed is increased. When the saddle is advanced to increase the command voltage 0 so that 0 0,, 0, the positive differential potential is applied to the amplifier A the amplified output of which actuating one of the polarized relays CR since at this time the direction-discriminating switch D is positioned so as to connect the relay CR while disconnecting the other relay CR As the result, the servo motor SM rotates forwardly to rotate the speed change gear shaft K in a direction to increase the speed of the shaft of the speed change gear. It will be understood that such rotation of the shaft K at the same time moves the sliding terminal b in a direction to make the difference 0 0 zero. In this case, if the feedback voltage 0,, is increased excessively to exceed the command voltage 0,, reversing the polarity of the amplifier output, the current flow through the polarized relay CR is also reversed in direction so that the contacts CR is not closed. It will be understood that on this occasion the discriminating switch D is in a position to connect the relay CR since the saddle is being advanced. Consequently, the servo motor SM stops to rotate interrupting the function of the control system. Thus, it is noted that the servo-motor SM is con fined so as to effect rotation only in one directionthereby preventing the system from falling in a state of hunting. As the saddle continues to advance, the command voltage 0 is increased gradually to go beyond a point where 6 0,,=O again exceeding the feedback voltage 0 to cause the control system again to start functioning.

Thus, it will be appreciated that when the saddle is advanced so that mand and feedback voltages varying in the direction of 3 ordinate y in the graph or FIGURE 3. On the other hand, when 0 0, the control system does not function with the voltages held substantially constant to form a stepped follow-up characteristic as shown in FIGURE 3.

Similarly, when the saddle is retreated, the directiondiscriminating switch D is turned to connect the polarized relay CR to allow the servo motor to rotate in the reverse direction thus to give the similar follow-up characteristic involving no hunting.

Thus it will be appreciated that, in accordance with the present invention, as the saddle is advanced toward the spindle axis of the lathe, the spindle rotation is successively increased to permit facing work to be performed at a substantially equal cutting speed. In general, in an automatic control system of this kind having no discriminating means, though the feedback voltage tends to follow the command voltage, the differential potential 6 -0 is not immediately reduced to zero but is first reduced in excess up till 0 -0 0 to cause the servo motor to rotate in the reverse direction again in excess to give 0 0 0, whereupon the'servo motor starts to effect forward rotation in endeavoring to realize the state 0 0 =0 thus giving rise to hunting. Therefore, it has been impossible to obtain a high sensitivity together with a substantial stability. In contrast to this, it will be appreciated that according to the present invention, since the direction-(iiscriminating switch D serves to preclude hunting, a precise servo system can be provided which forms a control system having a substantially high sensitivity.

What is claimed is:

I 1. A control system comprising first generating means for generating a variable input signal 0 means forindicating the polarity of the change de of said variable input signal, second generating means for generating a variable output signal 6 third genera-ting means responsive to said first and second generating means to generate a difference signal 0 -41 means responsive to said indicating means and said third generating means for energizing said second generating means to change said variable output voltage only when d0, and 0 -6,, have the same polarity.

2. A control system comprising first generating means for generating a variable input voltage, means for in- .dicating whether said variable input voltage is increasing or decreasing, second generating means for generating a variable output voltage, means comparing said input voltage with said output voltage to generate a control voltage, means responsive to said indicating means and said comparing means for activating said second generating means to change said variable output voltage only when either said input voltage is increasing and said input voltage is greater than said output voltage or said input voltage is decreasing and said input voltage is less than said output voltage.

3. A control system comprising first generating means for generating a variable input voltage, means for indicating whether said variable input voltage is increasing or decreasing, second generating means for generating a variable output voltage, means comparing said input voltage with said output voltageto generate a control voltage, means responsive to said indicating means and said comparing means for activating said second generating means to increase said variable output voltage only when said input voltage is increasing and said input voltage is greater than said output voltage, and to decrease said variable output voltage when said input voltage is decreasing and said input voltage is less than said output voltage.

4. A control system for controlling the speed of a first element driven by a source of mechanical power in accordance with the position and direction of movement of a second element, said control system comprising first and second controllably variable voltage sources having outputs, a differ-ential amplifier having input means and output means for indicating by the polarity of the output current the sign of the difference of two voltages applied to the input means, means for electrically connecting the outputs of said controllably variable voltage sources to the input means of said differential amplifier, said first voltage source being responsive to the position of said second element to generate a voltage whose amplitude is related to said position, a switch having electrical input and output means, means for connecting the input means of said switch to the output means of said differential amplifier, first and second polarized relays having coils and contact sets, means for connecting said coils to the output means of said switch, means for mechanically connecting said switch to said second element whereby when said second element is being moved in a first direction the coil of said first relay is connected to the output means of said differential amplifier and when said second element is being moved in a second direction the coil of said second relay is connected to the output means of said differential amplifier, said first relay being polarized to be energized only by a current flowing in a first direction in the output means of said differential amplifier, said second relay being polarized to be energized only by a current flowing in the opposite direction in the output means of said differential amplifier, a source of electrical power, a servo-motor, means for connecting said source of electrical power via the contact sets of said polarized relays in such a manner that when said first polarized relay is energized said servomotor rotates in one direction and when said second polarized relay is energized said servomotor rotates in the opposite direction, means for causing said source of mechanical power to be under control of said servomotor and means for causing said second voltage source to be responsive to the rotation of said servomotor to generate a voltage Whose amplitude is related to said rotation.

5. A control system for controlling the movement of a first element driven by a sourceof mechanical power in accordance with the position and direction of movement of a second element, said control system comprising first and second controllably variaible voltage sources having outputs, a differential amplifier having input means and output means for indicating by the polarity of the output current the sign of the difference of two voltages applied to the input means, means for electrically con necting the outputs of said controllably variable voltage sources to the input means of said differential amplifier, said first voltage source being responsive to the position of said second element to generate a voltage whose amplitude is related to its position, a switch having electrical input and output means, means for connecting the input means of said switch to the output means of said differential amplifier, first and second switching means, means for connecting said switching means to the output means of said switch, means for mechanically connecting said switch to said second element whereby when said second element is being moved in a firs-t direction said first switching means is made responsive to the output means of said differential amplifier and when said second element is being moved in a second direction said second switching means is made responsive to the output means of said differential amplifier, said first switching means being energizable only by a current flowing in a first direction in the output means of said difierential amplifier, said second switching means being energizable only by a current flowing in the opposite direction in the output means of said differential amplifier, a source of electrical power, a servomotor, means for connecting said source of electrical power via said first and second switching means in such a manner that when said first switching means is energized said servomotor rotates in one direction and when said second switching means is energized said serv'o- 7 motor rotates in the opposite direction, means for causing said source of mechanical power to be under control of said servomotor and means for causing said second voltage source to be responsive to the rotation of said servomotor to generate a voltage whose amplitude is related to said rotation.

6. A control system for controlling the speed of rotation of a lathe spindle driven by a source of rotary power in accordance with the position and direction of movement of a lathe saddle, control system comprising first and second controllably variable voltage sources having outputs, a difierential amplifier having input means and output means for indicating by the polarity of an output current the sign of the algebraic sum of two voltage-s applied to the input means, means for electrically connecting the outputs of said controllably variable voltage sources to said input means, said first voltage source being responsive to the position of said lathe saddle to generate a voltage whose amplitude is related to said position, a switch having electrical input and output means, means for connecting the input means of said switch to the output means of said amplifier, said switch means for mechanically connecting said switch to said lathe saddle whereby when said lathe saddle is being advanced the coil of said first relay is connected to the output means of said amplifier and when said lathe saddle is being withdrawn the coil of said second relay is connected to the output means of said amplifier, first and second polarized relays each having a coil and contact sets, means for connecting said coils to said output means of said first relay being polarized to be energized only when current flows in a first direction from the output means of said amplifier, said second polarized relay being energized to operate only when current flows in an opposite direction from the output means of said amplifier, a source of electrical power, a servomotor, means for connecting said source of electrical power via said contact sets to said servomotor in such a manner that when said first polarized relay is energized said servomotor rotates in one direction and when said. second polarized relay is energized said servomotor rotates in the opposite direction, spindle drive means responsive to the rotation of said servomotor for controlling the speed ofrot-ation of said spindle and said second voltage source being responsive to the rotation of said servomotor to generate a voltage whose amplitude is related to said rotation.

7. A control system for controlling the speed of rotation of a lathespindle driven by a source of rotary power in accordance with the positionand direction of movement of a lathe saddle, control system comprising first and second otentiometers, means for applying a voltage across said potentiometers, a differential amplifier having input means and output means for indicating by the polarity of an output current the sign of the algebraic sum of two voltages applied to the input means, means for electrically connecting the moving contact of said first potentiometer to said input means, means for electrically connecting the moving contact of said second potentiometer to said input means, means for mechanically coupling said lathe saddle to the moving contact of said first potentiometer, a direction-discriminating switch having one moving contact and first and second fixed contacts, means for mechanically connecting said moving contact to said lathe saddle whereby when said lathe saddle is being advanced said moving contact is positioned against said first fixed contact and when said lathe saddle is being withdrawn said moving contact is positioned against said secondfixed contact, first and second polarized relays each having a coil and contact sets, means for connecting one end of each of said coils to said output means of said differential amplifier, means for connecting the other end of said first coil to said first 6 fixed contact, means for connecting the other end of said second coil to said second fixed contact, said first relay being polarized to be energized only when current flows in a direction from the junction of said coils to said first fixed contact, said second polarized relay being energized to operate only when current flows in a direction from said second fixed contact to the junction of said coils, a source of electrical power, a servomotor,

means for connecting said source of electrical power via said contact sets to said servomotor in such a manner that when said first polarized relay is energized said servomotor rotates in one direction and when said second polarized relay is energized said servomotor rotates in the opposite direction, said source of rotary power responsive to the rotation of said servomotor for controlling the speed of rotation of said spindle and means for mechanically connecting the moving contact of said second potentiometer to said servomotor.

8. A control system for controlling the speed of a lathe spindle driven by a source of rotary power in accordance with the position and direction of movement of a lathe saddle, said control system compnising first and second controllably variable voltage sources having outputs, a differential amplifier having input means and output means, means for electrically connecting the outputs of said controllably variable voltage sources to the input means of said difierential amplifier, said difierential amplifier transmitting a signal of a first polarity when the voltage at the output of said first voltage source is greater than the voltage at the output of said second voltage source and transmitting a signal of a second polarity when the voltage at the output of said first voltage source is less than the voltage at the output of said second voltage source, said first voltage source being responsive to the position of said lathe saddle to generate a voltage whose amplitude is related to said position, a switch having electrical input and output means, means for connecting the input means of said switch to the output means of said diiferential amplifier, first and second polarized relays having coils and contact sets, means for connecting said coils to the output means of said switch, means for mechanically connecting said switch to said lathe saddle whereby when said lathe saddle is being advanced the coil of said first relay is connected to the output means of said differential amplifier and when said lathe saddle is being withdrawn the coil of said second relay is connected to the output means of said differential amplifier, said first relay being polarized to be energized only when a signal of said first polarity is transmitted by the output means of said differential amplifier, said second.

relay being polarized to be energized only when a signal of said second polarity is transmitted by the output means of said differential amplifier, a source of elec trical power, a servomotor, means for connecting said source of electrical power via the contact sets of said polarized relays in such a manner that when said first polarized relay is energized said servomotor rotates in one direction and when said second polarized relay is energized said servomotor rotates in the opposite direction, means for causing said source of rotary power to be under control of said servomotor and means for causing said second voltage source to be responsive to the rotation of said servomotor to generate a voltage whose amplitude is related to said rotation.

' References Cited in the file of this patent UNITED STATES PATENTS 2,534,801 Siltamaki Dec. 19, 1950 2,632,872 Warsher Mar. 24, 1953 2,774,928 Johnson et al. Dec. 18, 1956 

4. A CONTROL SYSTEM FOR CONTROLLING THE SPEED OF A FIRST ELEMENT DRIVEN BY A SOURCE OF MECHANICAL POWER IN ACCORDANCE WITH THE POSITION AND DIRECTION OF MOVEMENT OF A SECOND ELEMENT, SAID CONTROL SYSTEM COMPRISING FIRST AND SECOND CONTROLLABLY VARIABLE VOLTAGE SOURCES HAVING OUTPUTS, A DIFFERENTIAL AMPLIFIER HAVING INPUT MEANS AND OUTPUT MEANS FOR INDICATING BY THE POLARITY OF THE OUTPUT CURRENT THE SIGN OF THE DIFFERENCE OF TWO VOLTAGES APPLIED TO THE INPUT MEANS, MEANS FOR ELECTRICALLY CONNECTING THE OUTPUTS OF SAID CONTROLLABLY VARIABLE VOLTAGE SOURCES TO THE INPUT MEANS OF SAID DIFFERENTIAL AMPLIFIER, SAID FIRST VOLTAGE SOURCE BEING RESPONSIVE TO THE POSITION OF SAID SECOND ELEMENT TO GENERATE A VOLTAGE WHOSE AMPLITUDE IS RELATED TO SAID POSITION, A SWITCH HAVING ELECTRICAL INPUT AND OUTPUT MEANS, MEANS FOR CONNECTING THE INPUT MEANS OF SAID SWITCH TO THE OUTPUT MEANS OF SAID DIFFERENTIAL AMPLIFIER, FIRST AND SECOND POLARIZED RELAYS HAVING COILS AND CONTACT SETS, MEANS FOR CONNECTING SAID COILS TO THE OUTPUT MEANS OF SAID SWITCH, MEANS FOR MECHANICALLY CONNECTING SAID SWITCH TO SAID SECOND ELEMENT WHEREBY WHEN SAID SECOND ELEMENT IS BEING MOVED IN A FIRST DIRECTION THE COIL OF SAID FIRST RELAY IS CONNECTED TO THE OUTPUT MEANS OF SAID DIFFERENTIAL AMPLIFIER AND WHEN SAID SECOND ELEMENT IS BEING MOVED IN A SECOND DIRECTION THE COIL OF SAID SECOND RELAY IS CONNECTED TO THE OUTPUT MEANS OF SAID DIFFERENTIAL AMPLIFIER, SAID FIRST RELAY BEING POLARIZED TO BE ENERGIZED ONLY BY A CURRENT FLOWING IN A FIRST DIRECTION IN THE OUTPUT MEANS OF SAID DIFFERENTIAL AMPLIFIER, SAID SECOND RELAY BEING POLARIZED TO BE ENERGIZED ONLY BY A CURRENT FLOWING IN THE OPPOSITE DIRECTION IN THE OUTPUT MEANS OF SAID DIFFERENTIAL AMPLIFIER, A SOURCE OF ELECTRICAL POWER, A SERVO-MOTOR, MEANS FOR CONNECTING SAID SOURCE OF ELECTRICAL POWER VIA THE CONTACT SETS OF SAID POLARIZED RELAYS IN SUCH A MANNER THAT WHEN SAID FIRST POLARIZED RELAY IS ENERGIZED SAID SERVOMOTOR ROTATES IN ONE DIRECTION AND WHEN SAID SECOND POLARIZED RELAY IS ENERGIZED SAID SERVOMOTOR ROTATES IN THE OPPOSITE DIRECTION, MEANS FOR CAUSING SAID SOURCE OF MECHANICAL POWER TO BE UNDER CONTROL OF SAID SERVOMOTOR AND MEANS FOR CAUSING SAID SECOND VOLTAGE SOURCE TO BE RESPONSIVE TO THE ROTATION OF SAID SERVOMOTOR TO GENERATE A VOLTAGE WHOSE AMPLITUDE IS RELATED TO SAID ROTATION. 